In LTE-A, wider bandwidth, up to 100 MHz, is used to satisfy higher data rate requirement. For backward compatibility, the 100 MHz bandwidth is separated into multiple carrier components, each of which has the maximum width of 20 MHz. Therefore, each user may support up to 5 carrier components. For each user equipment (UE), there is one primary carrier component (PCC) and optionally one or more secondary carrier components (SCC), with the former always keeping active. Another related concept is primary cell (Pcell) and secondary cell (Scell) adopted by 3GPP, wherein a primary cell refers to a downlink/uplink PCC pair, and a secondary cell refers to a downlink/uplink SCC pair or a single downlink carrier. A primary cell is established by R8 radio resource control (RRC) connection procedure, and a secondary cell is established by new Release-10 secondary cell adding message. It has been agreed that each secondary cell will be assigned with one cell index to identify its corresponding downlink and uplink carrier components or corresponding downlink carrier.
Cross-scheduling has been agreed for LTE-A carrier aggregation so that physical downlink control channel (PDCCH) in one carrier component can indicate resource information of other carrier component(s). For this purpose, a carrier identification field (CIF) is inserted into PDCCH to indicate target carrier component(s) at which the resource information is located. One basic consensus on CIF is that CIF and cell index should be defined separately. Another consensus is that the downlink/uplink carrier components of the same cell linked by system information block 2 (SIB2) should be scheduled by the same downlink carrier component. And it is still inconclusive about CIF assignment and scheduling.
A solution of adopting implicit CIF assignment for each carrier component has been proposed. For example, the CIF is determined on the basis of the cell index of a carrier component or frequency information of a carrier component. Such kind of implicit assignment helps to save signaling overhead, but the gain is very small since at most 15 bits can be saved even in a case where a UE is configured with 5 carrier components. On the other hand, this implicit CIF assignment based on cell index violates the current consensus that CIF and cell index should be defined separately. Another drawback is that this solution may probably cause specification complexity when the CIF of a carrier component is reconfigured (e.g., the CIF of a carrier component is reconfigured from one PDCCH of a downlink carrier component to another) and also restrict reconfiguration flexibility (e.g., the network may need to assign a particular CIF value for a carrier component, instead of any cell order or frequency order). Another drawback of this solution is that potential loss of synchronization may happen with the CIF of the carrier components between an evolved Node B (eNB) and a UE. For example, one possibility for primary cell index assignment is to reserve a default value for the primary cell. Hence for primary cell change due to reconfiguration where the current primary cell is changed into a new secondary cell, the cell index of the primary cell needs to be changed, and its CIF should also be changed according to this implicit solution, thereby giving the chance of mismatch of CIF value of carrier components between the eNB and the UE. In addition, this implicit assignment method must further define a specific mapping strategy between CIF and cell index or frequency information, thereby increasing specification complexity is caused.